The present invention relates to foundry tooling processes and methods for casting operations. More particularly, the present invention relates to forming a core assembly useful in casting operations known in the art as "full mold" casting.
The use of the full mold casting process can provide improved mold utilization, i.e., getting more castings per mold use, reduced sand preparation and handling, and reduced casting cleaning costs. Generally, the process involves the use of a gasifiable plastic pattern formed in the shape of the article to be cast. The pattern is located in a foundry mold and surrounded by unbonded sand. Usually, the sand is aerated when the pattern is first inserted into the mold. Special aerating equipment is thus required. When molten metal is cast into the mold, the metal vaporizes the pattern, thereby assuming its configuration as the cast article.
Problems are encountered with the full mold process when it is desired to make large and/or long and thin castings, such as crank shafts or transmission cases. Because of the physical characteristics of the plastic pattern, it undergoes significant deformation and bending when sand is packed around it in the foundry mold. Also, with intricate and irregularly shaped patterns it is difficult to fill unbonded sand in all required areas. Other malformations occur in the plastic pattern between the time it is formed and the time the metal is cast due to significant dimensional changes in the pattern, such as by shrinkage of the plastic material. These problems have therefore made it impractical to utilize the full mold technique for large and/or long and thin castings.
Various methods have been suggested by those in the art to strengthen molds. For example, U.S. Pat. No. 4,043,379 discloses a "lost wax" casting process in which a wax pattern is formed in the configuration of the article to be cast, the wax form is surrounded by a hardenable material, the wax is melted out to leave a void, and molten metal is cast into the void to form the casting. This patent discloses molding the wax form about rigid reinforcing members which are removed after the wax is melted out. However, that process requires that the pattern be removed prior to casting of the metal.
U.S. Pat. No. 3,889,737 discloses filling a plastic pattern with loose sand which is then compacted to add stability to the pattern. This method, however, is applicable only to patterns in configurations which will hold loose sand; moreover, compacting the sand may actually exacerbate deformations in the pattern. U.S. Pat. No. 4,291,739 also suggests the use of loose sand, but in this case a vacuum is applied to the mold to stabilize the pattern. This method, however, excludes the use of a pattern having a solid core positioned in the interior thereof, as in the present invention; also, the force of the vacuum applied must be regulated so that it does not deform the pattern.
U.S. Pat. No. Re. 31,488 discloses a particularly efficacious method for forming intricate castings using gasifiable plastic patterns. This patent shows a mold assembly including an inner bonded sand core, a gasifiable thermoplastic pattern, and an outer bonded sand core for use in metal casting.
None of the aforementioned patents adequately prevents deformation of the plastic pattern while also preventing dimensional changes in the pattern due to shrinkage of the plastic nor provides a solution to the problems extant with the possible use of the full mold casting process to large or long and thin castings.
In view of the foregoing, it is an object of the present invention to provide a method for stabilizing plastic patterns useful in metal casting.
It is also an object of the present invention to stabilize such patterns against flexural and distortional forces as well as against dimensional changes.
It is also an object of the present invention to provide a method for casting large or long and thin castings by means of the full mold process.
It is a further object of the present invention to provide a core assembly having a stabilized plastic pattern which is useful in metal casting.
It is still a further object of the present invention to provide a method for producing such core assemblies.
Yet another object of the present invention is to provide a method of casting metal castings by means of such core assemblies.